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                    <i class="far fa-calendar-minus fa-fw"></i>发布日期:&nbsp;&nbsp;
                    2020-03-16
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                <h2 id="异常"><a href="#异常" class="headerlink" title="异常"></a>异常</h2><p>指执行过程出现非正常情况,Java处理异常的方式是中断处理<br>Throwable 异常的根类<br>Error: extends Throwable,不能处理只能尽量避免<br>Exception: extends Throwable,由于使用不当导致,可以避免</p>
<span id="more"></span>

<p>&#x2F;&#x2F; Exception:编译期异常,进行编译时出现的问题<br>&#x2F;&#x2F; RuntimeException:运行期异常,运行过程中出现的问题</p>
<ol>
<li>psvm throws XXXException{}</li>
<li>try{<br>&#x2F;&#x2F;可能会出现异常的代码	<br> }catch(XXXException e){<br> &#x2F;&#x2F;异常的处理逻辑<br> e.printStackTrace();<br> }</li>
</ol>
<h2 id="异常的处理"><a href="#异常的处理" class="headerlink" title="异常的处理"></a>异常的处理</h2><p>&#x2F;&#x2F; throw关键字:在指定的方法中抛出指定的异常<br>使用格式:<br>throw new XXXException(“异常产生的原因”)</p>
<p>注意:</p>
<ol>
<li>throw关键字必须写在方法的内部</li>
<li>throw关键字后面new的对象必须是Exception或者Exception的子类对象</li>
<li>throw关键字抛出指定的异常对象,我们就必须处理这个异常对象<br> throw关键字后面创建的是RuntimeException或其子类对象时我们可以不处理,默认交给JVM处理(打印异常对象,中断程序)<br> throw关键字后面创建的是编译异常(写代码的时候报错),我们就必须处理这个异常,要么throw,要么try…catch</li>
</ol>
<p>public class Demo03Throw {<br>    &#x2F;*<br>        定义一个方法,获取数组指定索引处的元素<br>        参数:<br>        int[] arr<br>        int index<br>        我们首先必须对方法传递过来的参数进行合法性校验<br>        如果参数不合法,那么我们就必须使用抛出异常的方式,告知方法调用者,传递的参数有问题</p>
<pre><code>    NullPointerException是一个运行期异常,我们不用处理,默认交给JVM处理

 */
public static void main(String[] args) &#123;
</code></pre>
<p>&#x2F;&#x2F;		int[] arr &#x3D; null;<br>&#x2F;&#x2F;		getElement(arr,0);<br>        int[] arr1 &#x3D; new int[3];<br>        getElement(arr1, 3);</p>
<pre><code>&#125;

public static int getElement(int[] arr, int index) &#123;
    /*
        对传递过来的参数进行合法性校验
        如果arr时null,我们就抛出空指针异常,告知调用者,&quot;传递的数组的值是null&quot;

     */
    if (arr == null) &#123;
        throw new NullPointerException(&quot;传递的数组的值是null&quot;);

    &#125;

    /*
        如果index的范围不在数组索引范围类,就抛出数组索引越界异常,
        告知方法的调用者&quot;传递的索引超出了数组的使用范围&quot;
     */
    if (index &lt; 0 || index &gt; arr.length - 1) &#123;
        throw new ArrayIndexOutOfBoundsException(&quot;传递的索引超出了数组的使用范围&quot;);
    &#125;

    int ele = arr[index];
    return ele;
&#125;
</code></pre>
<p>}</p>
<p>&#x2F;&#x2F; Object非空判断<br>Objects类中的静态方法<br>public static <T> T requireNonNull(T obj):查看指定引用对象不是null<br>public static int getElement(int[] arr, int index) {<br>        &#x2F;&#x2F;不用自己写校验合法性的代码,可以简化代码<br>        Objecs.requireNonNull(obj,”传递的数组的值是null”);</p>
<pre><code>    int ele = arr[index];
    return ele;
&#125;
</code></pre>
<p>&#x2F;&#x2F; throws关键字: 异常处理的第一种方式,交给别人处理<br>    作用:<br>        当方法内部抛出异常对象的时候,我们必须处理这个异常对象<br>    使用格式:<br>    修饰符 返回值类型 方法名(参数列表) throws AAAException{ton,BBBException{ton…{<br>    throw new AAAException(“产生原因”);<br>    throw new BBBException(“产生原因”);<br>    …<br>    }</p>
<p>public class Demo05Throws {<br>    public static void main(String[] args) throws IOException {<br>        readFile(“c:\a.txd”);<br>        readFile(“d :\a.txt”);</p>
<pre><code>&#125;
/*
    定义一个方法,对传递的文件路径进行合法判断
    如果路径不是&quot;c:\\a.txt&quot;,就抛出文件找不到异常对象
    注意:FileNotFoundException是编译异常,抛出编译异常就必须处理
         可以使用throws继续声明抛出FileNotFoundException这个异常对象,让方法调用者处理
         FileNotFoundException extends IOException,直接声明父类异常即可
 */
public static void readFile(String filename) throws IOException &#123;
    if(!(&quot;c:\\a.txt&quot;).equals(filename))&#123;
        throw new FileNotFoundException(&quot;传递路径错误&quot;);
    &#125;
    /*
        如果传递路径不是.txt结尾
        那么我们就抛出IO异常对象,告知方法的调用者,文件后缀名不对

     */
    if(!filename.endsWith(&quot;.txt&quot;))&#123;
        throw new IOException(&quot;文件后缀名错误&quot;);
    &#125;
    System.out.println(&quot;路径正确,读取文件&quot;);

&#125;
</code></pre>
<p>}</p>
<p>&#x2F;&#x2F;  try…catch:异常处理的第二种方式,自己处理异常<br>    格式:<br>        try{<br>            可能产生异常的代码<br>        }catch(定义一个异常的变量,用来接受try中抛出的异常对象){<br>            异常的处理逻辑,怎么处理异常对象<br>            一班会把异常信息记录到一个日志中<br>        }<br>        …<br>        catch(异常类名 变量名){<br>        }</p>
<pre><code>注意:
    1. try中可能会抛出多个异常对象,那么就可以使用多个catch来处理这些异常对象
    2. 如果try中产生了异常,那么就会执行catch中的异常处理逻辑,执行完毕catch中的处理逻辑,继续执行try...catch后面的代码
       如果try中没有产生异常,那么就不会执行catch中异常的出路逻辑,执行完try中代码,继续执行try...catch后面的代码
       
</code></pre>
<p>public class Demo01TryCatch {<br>    public static void main(String[] args) {<br>        try{<br>            readFile(“c:\a.txd”);<br>        }catch(IOException e){&#x2F;&#x2F; try中抛出什么异常对象,catch就定义什么异常变量,用来接收这个异常对象<br>            System.out.println(“文件后缀名错误”);<br>        }<br>        System.out.println(“后续代码”);<br>    }<br>    public static void readFile(String filename) throws IOException {</p>
<pre><code>    if(!filename.endsWith(&quot;.txt&quot;))&#123;
        throw new IOException(&quot;文件后缀名错误&quot;);
    &#125;
    System.out.println(&quot;路径正确,读取文件&quot;);
&#125;
</code></pre>
<p>}</p>
<p>&#x2F;&#x2F; Throwable类中的3个异常处理方法<br>String getMessage() 返回此throwable的简短描述<br>String toString() 返回此 throwable 的详细消息字符串<br>void printStackTrace() JVM打印异常对象,默认此方法,异常消息最全面</p>
<pre><code>    try&#123;
        readFile(&quot;c:\\a.txd&quot;);
    &#125;catch(IOException e)&#123;// try中抛出什么异常对象,catch就定义什么异常变量,用来接收这个异常对象
</code></pre>
<p>&#x2F;&#x2F;			System.out.println(“文件后缀名错误”);<br>&#x2F;&#x2F;			System.out.println(e.getMessage());&#x2F;&#x2F;文件后缀名错误<br>&#x2F;&#x2F;			System.out.println(e.toString());&#x2F;&#x2F;java.io.IOException: 文件后缀名错误<br>            e.printStackTrace();&#x2F;&#x2F;最全面<br>        }		</p>
<p>&#x2F;&#x2F; finally代码块<br>        try{<br>            可能产生异常的代码<br>        }catch(定义一个异常的变量,用来接受try中抛出的异常对象){<br>            异常的处理逻辑,怎么处理异常对象<br>            一班会把异常信息记录到一个日志中<br>        }<br>        …<br>        catch(异常类名 变量名){<br>        }finally{<br>            无论是否出现异常都会执行<br>        }<br>        注意:<br>            1. finally不能单独使用,必须和try一起使用<br>            2. finally一般用于资源释放(资源回收),无论程序是否出现异常,最后都要资源释放</p>
<pre><code>public static void main(String[] args) &#123;
    try&#123;
        readFile(&quot;c:\\a.txd&quot;);
    &#125;catch(IOException e)&#123;
        e.printStackTrace();
    &#125;finally&#123;
        System.out.println(&quot;资源释放&quot;);
    &#125;
&#125;

如果finally有return语句,永远返回finally中的结果,避免该情况,所以尽量不要在finally里面写return
</code></pre>
<p>&#x2F;&#x2F; 多异常的捕获处理</p>
<ol>
<li>多个异常分别处理:<br> 有一个异常用一次try…catch</li>
<li>多个异常一次捕获,多次处理<br> 一个try多个catch<br> 注意:catch里面定义的异常变量,如果有子父类关系,那么子类的异常变量必须写在上面,否则会报错</li>
<li>多个异常一次捕获一次处理<br> catch父类<br> try{}catch(Exception e){}</li>
</ol>
<p>&#x2F;&#x2F; 子父类异常<br>如果父类抛出了多个异常,子类重写父类方法时,抛出和父类相同的异常或者是父类异常的子类或者不抛出异常<br>父类方法没有抛出异常,子类重写父类该方法时也不可以抛出异常,此时子类产生该异常,只能捕获处理,不能声明抛出<br>注意: 父类异常时什么样,子类异常就什么样<br>public class Fu {<br>    public void show01() throws NullPointerException,ClassCastException{}<br>    public void show02() throws IndexOutOfBoundsException{}<br>    public void show03() throws IndexOutOfBoundsException{}<br>    public void show04(){}<br>    public void show05(){}</p>
<p>}<br>class Zi extends Fu{<br>    &#x2F;&#x2F;重写父类方法时抛出和父类相同的异常<br>    public void show01() throws NullPointerException,ClassCastException{}<br>    &#x2F;&#x2F;或者抛出父类异常的子类<br>    public void show02() throws ArrayIndexOutOfBoundsException{}<br>    &#x2F;&#x2F;或者不声明异常<br>    public void show03(){}</p>
<pre><code>//父类没有抛出异常,子类也不能抛出异常
public void show04()&#123;&#125;
//如果产生该异常,只能捕获处理,try...catch
public void show05()&#123;
    try &#123;
        throw new Exception(&quot;编译器异常&quot;);
    &#125; catch (Exception e) &#123;
        e.printStackTrace();
    &#125;
&#125;
</code></pre>
<p>}</p>
<h2 id="自定义异常"><a href="#自定义异常" class="headerlink" title="自定义异常"></a>自定义异常</h2><p>自定义一个注册异常<br>&#x2F;*<br>    自定义异常类<br>    格式:<br>    public class XXXException extends Exception&#x2F;RuntimeException{<br>        添加一个空参数的构造方法<br>        添加一个带异常信息的构造方法<br>    }<br>    注意:<br>        1. 自定义异常类一班都是以Exception结尾,说明是一个异常类<br>        2. 必须继承Exception&#x2F;RuntimeException<br> *&#x2F;<br>public class RegisterException extends Exception {<br>    &#x2F;&#x2F;添加一个空参数的构造方法</p>
<pre><code>public RegisterException() &#123;
    super();
&#125;

/*
    添加一个带异常信息的构造方法
    所有异常类都会有一个带异常信息的构造方法,方法内部会调用父类异常信息的构造方法,让父类来处理这个异常信息
 */
public RegisterException(String message) &#123;
    super(message);
&#125;
</code></pre>
<p>}</p>
<p>&#x2F;*<br>    要求:模拟注册操作,如果用户名已注册,则抛出异常,并提示:该用户名已被注册<br>    分析:<br>    1. 使用数组保存已经注册过的用户名(数据库)<br>    2. 使用Scanner获取用户输入的注册用户名<br>    3. 定义一个方法,对用户输入中注册的用户名进行判断<br>        遍历存储已经注册过用户名的数组,获取一个用户名<br>        使用获取到的用户名和用户名比较<br>        true:已存在,抛出异常<br>        false:继续遍历比较<br>        循环结束没有找到重名,注册成功</p>
<p> *&#x2F;<br>public class Demo01RegisterException {<br>    &#x2F;&#x2F;1. 使用数组保存已经注册过的用户名<br>    static String[] usernames &#x3D; {“张三”,”李四”,”王八”};</p>
<pre><code>public static void main(String[] args) throws RegisterException &#123;
    //2. Scanner获取
    Scanner sc = new Scanner(System.in);
    System.out.println(&quot;请输入要注册的用户名:&quot;);
    String username = sc.next();
    checkUsername(username);
&#125;

//3. 定义判断方法
public static void checkUsername(String username) throws RegisterException &#123;
    //遍历存储用户名
    for (String name : usernames) &#123;
        if(name.equals(username))&#123;
            throw new RegisterException(&quot;该用户名已被注册&quot;);
        &#125;
    &#125;
    System.out.println(&quot;注册成功&quot;);
&#125;
</code></pre>
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                    datas.forEach(function (data) {
                        var isMatch = true;
                        var data_title = data.title.trim().toLowerCase();
                        var data_content = data.content.trim().replace(/<[^>]+>/g, "").toLowerCase();
                        var data_url = data.url;
                        data_url = data_url.indexOf('/') === 0 ? data.url : '/' + data_url;
                        var index_title = -1;
                        var index_content = -1;
                        var first_occur = -1;
                        // only match artiles with not empty titles and contents
                        if (data_title !== '' && data_content !== '') {
                            keywords.forEach(function (keyword, i) {
                                index_title = data_title.indexOf(keyword);
                                index_content = data_content.indexOf(keyword);
                                if (index_title < 0 && index_content < 0) {
                                    isMatch = false;
                                } else {
                                    if (index_content < 0) {
                                        index_content = 0;
                                    }
                                    if (i === 0) {
                                        first_occur = index_content;
                                    }
                                }
                            });
                        }
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                        if (isMatch) {
                            str += "<li><a href='" + data_url + "' class='search-result-title'>" + data_title + "</a>";
                            var content = data.content.trim().replace(/<[^>]+>/g, "");
                            if (first_occur >= 0) {
                                // cut out 100 characters
                                var start = first_occur - 20;
                                var end = first_occur + 80;
                                if (start < 0) {
                                    start = 0;
                                }
                                if (start === 0) {
                                    end = 100;
                                }
                                if (end > content.length) {
                                    end = content.length;
                                }
                                var match_content = content.substr(start, end);
                                // highlight all keywords
                                keywords.forEach(function (keyword) {
                                    var regS = new RegExp(keyword, "gi");
                                    match_content = match_content.replace(regS, "<em class=\"search-keyword\">" + keyword + "</em>");
                                });

                                str += "<p class=\"search-result\">" + match_content + "...</p>"
                            }
                            str += "</li>";
                        }
                    });
                    str += "</ul>";
                    $resultContent.innerHTML = str;
                });
            }
        });
    };

    searchFunc('/shiqing/search.xml', 'searchInput', 'searchResult');
});
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